The photoelectric type pulse signal detection is extensively applied to the field of medical care and consumer electronics, and particularly applied to wearable devices due to its characteristics such as convenient use, small size and low power consumption.
A main principle of photoelectric type pulse signal detection is that when light is irradiated on skin, it is partly absorbed and partly reflected; when heart beats, a flow rate of arterial blood in arterial blood vessels presents a periodic motion identical with a heartbeat cycle, which causes cyclic changes of the intensity of the reflected light. A photoelectric transmitter transmits light beams to the skin, the light beams are reflected by the skin and received by a photoelectric receiver, and a cycle and frequency of heartbeat may be obtained according to a change tendency of the intensity of the reflected light.
In practical application, the photoelectric type pulse detection is usually confronted with interference from ambient light. Natural light and artificial light outside a photoelectric sensor is irradiated on the photoelectric receiver to form ambient light interference. In practical application, the photoelectric type pulse detection are further vulnerable to the interference of a user's actions. For example, when it is applied to a smart watch or smart wristband, it is susceptible to the interference of a hand's motion. When it is applied to a smart earphone, it is susceptible to the interference of the head motions and respiration. Relative motion between a photoelectric pulse measuring instrument and skin causes an optical transmission pathway between the photoelectric transmitter and photoelectric sensor to change, causes changes of light intensity, and forms motion interference. The resultant ambient light interference and/or motion interference affect signal energy statistics of the reflected light and thereby affect a heartbeat cycle detection precision.
In conventional technologies, a customary method of eliminating and weakening ambient light interference is to improve the structure and minimize a gap between the optical sensor and the external environment to block the ambient light; another customary method is to strengthen a transmission intensity of a light source. However, these methods are less applicable in a portable device and a wearable device. In such devices, for the sake of convenience and comfort in use, contact between the sensor and skin is not tight, a gap is apt to occur, and a size of the gap varies with relative motions so that the ambient light is hard to effectively block; in a wearable device, the apparatus needs to operate in a longer time period, so increasing the light source intensity will cause excessive power consumption, and shorten the use time period, which is not applicable.
In conventional technologies, a customary method of eliminating and weakening motion interference is to strengthen the clamp or contact of the photoelectric sensor and skin to eliminate the motion interference. However, for a wearable device, the sensor cannot tightly contact with the skin and the body motion is frequent, so this method is not practical; another customary method is using an accelerometer to detect actions so as to eliminate action interference in the photoelectric sensor signal. However, since the accelerometer and photoelectric sensor are different types of sensors, different signal collecting systems need to be configured, and rigid sample cycle consistency is required between the signal collecting systems. The correspondence relationship between the accelerometer signal and the optical signal is complicated so that complexity of the hardware and software is high.